Conventional telephony services are based upon the provision of a dedicated circuit to connect two telephony users via an exchange. The set-up and maintenance of the circuit have an associated cost, but the dedicated circuit allows a guaranteed quality of service (QoS) for the telephony service provided over the circuit. In contrast, computer communications made over packet based networks do not involve the set-up and maintenance of a circuit, but instead a stream of packets are routed from one computer to a second computer. The route taken by consecutive packets, may vary according to changes in network performance, congestion, etc. One of the disadvantages of packet-based networks when compared with circuit-based networks is that there is no guarantee that all of the packets transmitted by a computer will arrive at the destination computer or that those packets that are received by the destination computer will arrive in the same order that they were transmitted. Packets can be re-transmitted following a prompt by the destination computer and the application should be capable of re-ordering the received packets into the correct sequence and these steps are of little consequence if the data being transmitted between the computers does not have any time dependence., for example a file transfer, the transmission of an email message, etc. However, if the computers are transmitting real-time data, such as telephony or audio-visual signals for example, then the reception of out of sequence packets and the failure to receive some packets in time, or not at all, may cause significant problems with the playback of the real-time data.
Despite these disadvantages, it is perceived that in the near future most high capacity communications networks will be packet-based networks which will carry all forms of data, including real-time data such as speech and video. It is currently possible to use the public Internet to carry telephony services (often referred to as computer telephony (CT), PC telephony, Internet telephony or voice over IP (VoIP)) but the relatively low bandwidth available to Internet users means that such services tend to be of low quality.